The present invention is related to a method of measuring fluid velocity using a LDV and, more particularly, to a method which uses two linear differential arrays of photo detectors.
The measurement of localized fluid velocity by the laser Doppler method has developed along classical heterodyne techniques. Such methods involve mixing of the Doppler-shifted scattered laser beam with a reference beam using a conventional heterodyne method. The beat frequency of the scattered beam and the reference beam is then measured to determine the velocity of the scatterer and thus the fluid velocity. A dual-scatter or dual-differential Doppler method was proposed later in which the scattering particle is illuminated by two beams derived from the same laser and mixing is accomplished only between the scattered light. This mode has been preferred because of superior signal-to-noise (S/N) ratio and the ease of alignment in most practical situations. A conventional Laser Doppler Velocimeter (LDV) uses a single photosensor as the optical mixer, detecting the full power of optical signals. However, in both of these methods, there exists an uncertainty as to the direction of motion of the scatterer. Rotating radial gratings or Bragg cells have been used to resolve this directional ambiguity. These devices introduce a zero-velocity frequency offset in the linear frequency-to-velocity relation. Furthermore, Doppler pedestal (i.e. the presence of a constant level instead of Zero level) has to be removed by high-pass filtering the Doppler signal prior to signal processing on period counting electronics. It is thus desirable to have a simple technique for measuring the fluid velocity or the velocity of the scatterer therein which does not use complicated optical components such as Bragg cell or radial grating as mentioned above.